In a die-casting machine, a molten metal (melt) supplied to a sleeve is injected into a die cavity communicating with the sleeve by a plunger tip slidably movable in the sleeve, and solidified by cooling to produce a die-cast article. Accordingly, an inner surface of the sleeve is eroded by the melt, and worn by sliding with the plunger tip. When the inner surface of the sleeve is damaged by erosion and wear, a melt enters a gap between the sleeve and the plunger tip, and solidifies to increase the sliding resistance of the sleeve, resulting in a low injection speed and thus poor product quality. When a large amount of a lubricant is used to reduce sliding resistance between the sleeve and the plunger tip and to prevent seizure, the introduction of impurities such as a gas into the melt likely occurs, resulting in low product quality.
Proposed to reduce the erosion and wear of an inner surface of a sleeve is a die-casting sleeve having a composite structure, in which a ceramic-made inner cylinder is shrink-fit into a conventional outer cylinder made of a high-strength, low-thermal-expansion metal material. For example, JP 7-246449 A (Patent Reference 1) discloses a die-casting sleeve comprising an inner cylinder made of ceramics such as silicon nitride, sialon, etc. shrink-fit into an outer cylinder made of a high-strength, low-thermal-expansion metal material; the high-strength, low-thermal-expansion metal material having an average thermal expansion coefficient of 1×10−6/° C. to 5×10−6/° C. between room temperature and 300° C., and 5×10−6/° C. or more between room temperature and 600° C. This die-casting sleeve is provided at both ends of the outer cylinder with fixing rings made of hot work tool steel, and the inner cylinder is axially held by the fixing rings. Patent Reference 1 describes that with a high-strength, low-thermal-expansion metal material having such an average thermal expansion coefficient, sufficient shrink fitting can be conducted easily, without axial and circumferential displacement of the inner and outer cylinders.
In the die-casting sleeve of Patent Reference 1, however, when the fixing ring disposed in an injection-opening-side tip portion is not sufficiently cooled, for example, an inherently brittle ceramic-made inner cylinder in the injection-opening-side tip portion subjected to particularly high internal pressure may suffer damage such as local breakage and cracking.
JP 2002-192320 A (Patent Reference 2) discloses a die-casting sleeve comprising an inner cylinder fit in an outer cylinder, the inner cylinder coming into contact with a molten metal being constituted by a front member on the injection opening side and a rear member disposed on the rear side of the front member; the front member being formed by a metal material; and the rear member being formed by a ceramic material. Though Patent Reference 2 does not describe the details of the metal material, it describes as only one example an outer cylinder made of hot work die steel SKD61, a front member made of a composite metal material comprising 30% by volume of ceramic particles of silicon nitride dispersed in 70% by volume of an Fe—Ni alloy matrix, and a rear member made of sialon ceramics; the front member and the rear member being shrink-fit into the outer cylinder. It describes that by such a die-casting sleeve, the local damage of the tip portion can be avoided, thereby extending use life, and reducing the number of maintenance steps.
However, because the die-casting sleeve exemplified in Patent Reference 2 uses a material comprising low-thermal-expansion silicon nitride ceramic particles dispersed in an Fe—Ni alloy for the front member, the thermal expansion coefficient of the front member is smaller than that of the outer cylinder (hot work die steel SKD61) and larger than that of the rear member (sialon ceramics), so that shrink fitting is likely loosened between the front member and the rear member of the inner cylinder by temperature elevation during operation, resulting in a gap or a step between them. When an aluminum melt residing in the gap or step is solidified, a plunger tip hits the solidified aluminum during injection, so that the plunger tip and the inner cylinder are damaged. Because the rear member is made of ceramics, it is likely subjected to damage such as breakage, cracking, etc. in a boundary between the front member and the rear member.
Patent Reference 2 further describes that a boundary between the front member and the rear member is preferably located forward an aluminum-melt-supplying opening. For example, the die-casting sleeve shown in FIG. 1 comprises a metal front member extending to a position near the aluminum-melt-supplying opening. Because the metal front member has higher thermal conductivity than that of the ceramic-made rear member, it has lower capability of keeping the temperature of the aluminum melt, so that an aluminum melt is cooled inside the front member on the rear end side (near the supplying opening), likely forming solidified pieces, when the front member extends to a position near the supplying opening, at which the temperature is relatively low during operation. If the solidified pieces were introduced into die-cast products, cold flakes would be formed in the products, making the products defective. The cold flakes are formed when solidified pieces are introduced into products in a sleeve of a die-casting machine. Because the metal front member on the rear end side has a relatively low surface temperature during die-casting, cold flakes are likely formed in the products.